1. The Field of the Invention
The present invention relates to a skin thermocouple and in particular to a thermocouple providing improved diagnostic information on the operation of a heater.
2. The Prior Art
The basic purpose of a skin thermocouple is to protect a tube from overheating. An accurate skin thermocouple will also provide diagnostic information on the operation of the heater in which the tube is incorporated. All heaters which are susceptible to coke formation around the tubes should be provided with skin thermocouples. These types of heaters include all refinery heaters processing crude and heavier feed stocks. Also, heaters operating with tube wall temperatures near the maximum permissible temperature of the tube should be equipped with skin thermocouples.
The skin thermocouple is preferably located at the point of highest temperature on the tube for the obvious reason that this is the point where damage is most likely to occur. A good skin thermocouple must not only measure the temperature of the tube metal accurately, but also have a life span which is substantially equal to the turnaround time of the units, which time is usually in excess of two years.
The prior art is represented by U.S. Pat. No. 3,015,234 to Springfield; U.S. Pat. No. 3,263,502 to Springfield; U.S. Pat. No. 3,901,080 to Hilborn; and U.S. Pat. No. 4,625,200 to Hilborn. The first Springfield reference shows means for mounting a bimetallic heat sensing member in a heater chamber, such as a pressure vessel. The second Springfield reference concerns means for mounting multiple thermocouples into a device to be temperature measured. The first Hilborn reference considers the problems of mounting a skin temperature measuring device on a curved surface and includes a shield to protect the thermocouple junction. The second Hilborn reference relates to positioning a thermocouple cable in such a manner as to detect hot spots within an area to be measured. None of these references addresses the specific problem solved by the present invention.